Traveling wave tubes having backward wave suppressor devices

Abstract

There is disclosed a traveling wave tube with a slow wave structure having means for frequency and directionally sensitive wave amplification. The slow wave structure includes a conductive helix coaxially mounted within a conductive tubular housing and a conductive support structure longitudinally disposed within the housing and extending transversely from the helix to the housing. In some embodiments, the support structure is combshaped and in one embodiment is a helical ridge wound in registration with and in the same sense as the helix. The pitch of the housing and a preselected parameter of the support structure are simultaneously varied along the length of the helix such that a wave at a given frequency band traveling along the slow wave structure in a first direction is preferentially amplified with respect to waves traveling in an opposite direction. In various embodiments discussed, the first direction is the direction of travel of the electron beam, thereby providing a traveling wave tube which operates as a forward wave amplifier in which backward waves are suppressed.

I claim: 
     
       1. A traveling wave tube comprising: (a) means for providing an electron beam directed along an axis, said electron beam traveling in a first direction;   (b) an electrically conductive slow wave structure including (1) a helix disposed along and about said axis;   (2) tubular housing coaxially disposed about said helix; and   (3) a support structure for said helix;     (c) the pitch of said helix varying as a predetermined function of distance along said helix; and   (d) a preselected parameter of said support structure varying as a function of distance along said helix in a set relationship to said varying of said pitch of said helix such that for a first wave traveling along said slow wave structure having its phase velocity and group velocity along said first direction, the change in phase velocity caused by said varying of said pitch tends to cancel the change in phase velocity caused by said varying of said parameter of said support structure so as to result in a sufficiently uniform phase velocity as a function of distance along said slow wave structure to favor amplification of said first wave, whereas for a second wave traveling along said slow wave structure having its phase velocity along said first direction and its group velocity along the opposite direction, the change in phase velocity caused by said varying of said pitch is additive to the change in phase velocity caused by said varying of said parameter of said support structure so as to result in a sufficiently non-uniform phase velocity as a function of distance along said slow wave structure to suppress said second wave.   
     
     
       2. The device of claim 1 wherein said pitch of said helix increases in said first direction as said predetermined function of said distance along said helix. 
     
     
       3. The device of claim 1 wherein said pitch of said helix decreases in said first direction as said predetermined function of said distance along said helix. 
     
     
       4. The device of claim 1 wherein said support structure includes two comb shaped members extending in a direction parallel to the length of said helix and mounted substantially diametrically opposite to said helix within said tubular housing so as to form a longitudinal plane of symmetry containing said axis, each comb-shaped member having a spine portion and an array of axially spaced-apart fingers projecting from said spine. 
     
     
       5. The device of claim 4 wherein, in each of said comb shaped members, the tips of successive ones of the fingers are connected to respective ones of said successive turns of said helix, with said spine connected to said tubular housing. 
     
     
       6. The device of claim 5 wherein each tip of each of said fingers is longitudinally centered on and no wider than the width of the connected one of said successive turns of said helix. 
     
     
       7. The device of claim 5 wherein said preselected parameter of said support structure is the length of each of said fingers, said length of successive ones of said fingers varying along said length of said helix. 
     
     
       8. The device of claim 7 wherein said pitch of said helix increases as said predetermined function of said distance along said helix in said first direction and said length of successive ones of said fingers increases along said helix in said first direction. 
     
     
       9. The device of claim 7 wherein said pitch of said helix decreases as said predetermined function of said distance along said helix in said first direction, and said length of successive ones of said fingers decreases along said helix in said first direction. 
     
     
       10. The device of any one of claims 1 through 9 or 5 wherein said helix is provided with conduit means for the flow of a cooling fluid in thermal contact with said helix. 
     
     
       11. The device of claim 10 wherein said conduit means is a helix wound in the same sense as an integral part of said helix. 
     
     
       12. The device of any one of claims 1, 8 or 9 wherein said predetermined function of said distance along said helix is substantially a linear function. 
     
     
       13. The device of any one of claims 1, 8 or 9 wherein said predetermined function of said distance along said helix is substantially a cosine function, the total variation of the argument of said cosine being one-half cycle and the minimum and maximum amplitudes of said variation being at opposite ends of said helix. 
     
     
       14. The device of claims 8 or 9 wherein the total variation of said pitch between opposite ends of said helix is from six percent to 25 percent of the average pitch of said helix and the ratio of finger length variation to pitch variation is in a range from 1.1 to 1.7. 
     
     
       15. The device of claim 14 wherein said total variation of said pitch is approximately 10 percent and said ratio of said finger length variation to said pitch variation is approximately 1.3. 
     
     
       16. The device of claims 8 or 9 wherein the variation of said pitch follows substantially the same function as the variation of said length of said fingers. 
     
     
       17. The device of claim 6 wherein each of said tips of said fingers includes a concave portion conforming to the curvature of the outer periphery of said helix and said spine includes a convex portion conforming to the curvature of the inner periphery of said housing. 
     
     
       18. The device of claim 17 wherein a portion of each of said fingers define a pair of substantially mutually parallel edge surfaces which at their outer tips are coextensive with said spine and are substantially parallel to said longitudinal plane of symmetry. 
     
     
       19. The device of claims 17 or 18 wherein a portion of each of said fingers define a pair of edge surfaces extending radially outward from said helix and symmetrical to said longitudinal plane of symmetry. 
     
     
       20. The device of claim 19 wherein each pair of said radially disposed surfaces subtend an angle of substantially 90 degrees at said axis of said helix. 
     
     
       21. The device of claim 18 wherein said preselected parameter of said support structure is said thickness, where said thickness is defined as said perpendicular distance between said substantially parallel edge surfaces, said thickness on successive ones of said fingers varying along said length of said helix. 
     
     
       22. The device of claim 21 wherein said pitch of said helix increases as said predetermined function of said distance along said helix in said first direction, and said thickness of said successive ones of said fingers decreases along said helix in said first direction. 
     
     
       23. The device of claim 21 wherein said pitch of said helix decreases as said predetermined function of said distance along said helix in said first direction, and said thickness of said successive ones of said fingers increases along said helix in said first direction. 
     
     
       24. The device of claims 22 or 23 wherein said predetermined function of said distance along said helix is a linear function. 
     
     
       25. The device of claims 22 or 23 wherein said predetermined function of said distance along said helix is substantially a cosine function, the total variation of the argument of said cosine being one-half cycle, and the minimum and maximum amplitude of said variation being at opposite ends of said helix. 
     
     
       26. The device of claims 22 or 23 wherein the total variation of said pitch between opposite ends of said helix is in the range from 6 percent to 25 percent of the average pitch of said helix and the ratio of thickness variation to pitch variation is in the range from 1.8 to 2.8. 
     
     
       27. The device of claim 26 wherein said total variation of said pitch is approximately 13 percent and said ratio of thickness variation to pitch variation is approximately 2.3. 
     
     
       28. A traveling wave tube comprising: (a) means for providing an electron beam directed along an axis;   (b) an electrically conductive slow wave structure including (1) a helix disposed along and about said axis, and   (2) a support structure for said helix, said support structure comprising two comb shaped members extending in a direction parallel to the length of said helix and mounted substantially diametrically opposite to said helix so as to form a longitudinal plane of symmetry containing said axis, each comb shaped member having a spine portion and an array of axially spaced-apart fingers projecting from said spine, said fingers connected to the outer periphery of said helix, each finger, as viewed from a direction parallel to said axis, having side portions which diverge away from said helix and are symmetrical to said longitudinal plane of symmetry.     
     
     
       29. The device of claim 28 wherein said preselected parameter of said support structure is the transverse arcuate distance of said edges of said ridge portion. 
     
     
       30. The device of claim 28 wherein said preselected parameter of said support structure is the area, transverse to said axis, of each turn of said ridge portion. 
     
     
       31. The device of claim 28 wherein said preselected parameter of said support structure is the distance, substantially transverse to said axis, between each pair of said substantially opposed edge surfaces. 
     
     
       32. The device of claim 31 wherein said pitch of said helix increases as said predetermined function of said distance along said helix in said first direction, and said predetermined parameter of said support structure decreases as a function of distance along said length of said helix in said first direction. 
     
     
       33. The device of claim 31 wherein said pitch of said helix decreases as said predetermined function of said distance along said helix in said first direction, and said predetermined parameter of said support structure increases as a function of distance along said length of said helix in said first direction. 
     
     
       34. The device of claim 31 wherein said pitch of said helix increases as said predetermined function of said distance along said helix in said first direction, and said predetermined parameter of said support structure decreases as a function of distance along said length of said helix in said first direction. 
     
     
       35. The device of claim 31 wherein said pitch of said helix decreases as said predetermined function of said distance along said helix in said first direction, and said predetermined parameter of said support structure increases as a function of distance along said length of said helix in said first direction. 
     
     
       36. The device of claims 32, 33, 34 or 35 wherein said predetermined function is substantially a linear function. 
     
     
       37. The device of any one of claims 32, 33, 34 or 35 wherein said predetermined function is substantially a cosine, the total variation of the argument of said cosine being one-half cycle and the minimum and maximum amplitudes of said variation being at opposite ends of said helix. 
     
     
       38. The device of any one of claims 1, 7, 17, 28 or 29 wherein said traveling wave tube comprises a plurality of amplifying sections. 
     
     
       39. The device of any one of claims 1, 7, 16, or 28 wherein said helix is a monofilar helix. 
     
     
       40. The device of any ne of claims 1, 7, 17, 29 or 5 wherein said slow wave structure includes two axially interleaved helices, each wound in the same sense and having the same diameter so as to form a bifilar helix. 
     
     
       41. The device of claim 1 wherein a base portion defines said helix and a ridge portion extending substantially radially outwardly from said base portion defines said support structure. 
     
     
       42. The device of claim 41 wherein said ridge portion has a dimension in the direction of said axis which is less than the axial dimension of said base portion. 
     
     
       43. The device of claim 42 wherein said ridge portion is substantially longitudinally centered on said base portion. 
     
     
       44. The device of claims 28 or 42 wherein said preselected parameter of said support structure is a radial dimension of said ridge portion. 
     
     
       45. The device of claim 5 wherein each of said fingers, as viewed from a direction parallel to said axis, has sides which diverge away from said helix towards said housing. 
     
     
       46. The device of claim 18 wherein said preselected parameter of said support structure is a physical dimension of said finger taken at substantially right angles to said plane of symmetry. 
     
     
       47. The device of claims 22 or 23 wherein the variation of said pitch follows substantially the same function as the variation of said thickness of said finger. 
     
     
       48. The device of claim 41 wherein said preselected parameter of said support structure is a peripheral dimension of said ridge portion and peripheral portions of the ridge portion on each of successive turns of the helix are absent on longitudinally staggered but otherwise substantially radially opposed locations along the length of the helix so as to form pairs of substantially opposed edge surfaces of said ridge portion joined at their ends by substantially arcuate edges of said ridge portion. 
     
     
       49. A traveling wave tube comprising: (a) means for providing an electron beam directed along an axis;   (b) an electrically conductive slow wave structure including (1) a helix disposed along and about said axis; and   (2) a support structure comprising two arrays of axially spaced-apart fingers extending in a direction parallel to the length of said helix, each array mounted substantially diametrically opposite to said helix so as to form a longitudinal plane of symmetry containing said axis, the tips of successive ones of said fingers in each of said arrays being connected to respective ones of successive turns of said helix, each finger, as viewed from a direction parallel to said axis, having side portions which diverge away from said helix and are symmetrical to said longitudinal plane of symmetry.     
     
     
       50. The device of claims 48 or 49 wherein each finger has side portions which are substantially parallel to said plane of symmetry. 
     
     
       51. A traveling wave tube comprising: (a) means for providing an electron beam directed along an axis, said electron beam traveling in a first direction;   (b) an electrically conductive slow wave structure including (1) a helix disposed along and about said axis;   (2) a housing disposed about said helix; and   (3) a support structure for said helix;     (c) the pitch of said helix varying as a predetermined function of distance along said helix; and   (d) a preselected parameter of said support structure varying as a function of distance along said helix in a set relationship to said varying of said pitch of said helix such that for a first wave traveling along said slow wave structure having its phase velocity and group velocity along said first direction, the change in phase velocity caused by said varying of said pitch tends to cancel the change in phase velocity caused by said varying of said parameter of said support structure so as to result in a sufficiently uniform phase velocity as a function of distance along said slow wave structure to favor amplification of said first wave, whereas for a second wave traveling along said slow wave structure having its phase velocity along said first direction and its group velocity along the opposite direction, the change in phase velocity caused by said varying of said pitch is additive to the change in phase velocity caused by said varying of said parameter of said support structure so as to result in a sufficiently non-uniform phase velocity as a function of distance along said slow wave structure to suppress said second wave.